Method of forming polysilicon having a desired surface roughness

ABSTRACT

A method for reliably forming polysilicon of a desired surface roughness includes providing a layer of doped or undoped amorphous silicon on a substrate and heating said substrate while monitoring the emission of said substrate and comparing the monitored emission with an expected emission attributable to the heating regime employed. An increase in the monitored emission not attributable to the heating regime signals a transition of the layer of amorphous silicon to rough polysilicon. A decrease in the monitored emission not attributable to the heating regime signals a transition to smooth polysilicon. The increases and decreases in the monitored emission can be used to end the heating regime at the time at which the desired surface roughness of polysilicon is formed, or merely to passively monitor the process.

This application is a continuation of application Ser. No. 08/572,968,filed Dec. 15, 1995, now U.S. Pat. No. 5,688,550, which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to the manufacture of semiconductordevices. More particularly, the present invention is directed to amethod of forming polysilicon having a crystal structure with a desiredsurface roughness useful in the manufacture of semiconductor memorydevices.

2. The Relevant Technology

Polysilicon is used in a variety of ways in the design and fabricationof integrated circuits. Polysilicon is used in forming gates, contacts,capacitors, and many other circuit structures.

The capacitance of polysilicon is influenced significantly by itsstructure. The surface roughness of polysilicon must be consistentlycontrolled to achieve desired capacitance. Capacitance of capacitorsconstructed with polysilicon generally increases with surface roughnessbecause of increased surface area.

Consistency in achieving desired polysilicon surface roughness in theproduction fabrication environment has proved somewhat difficult.

SUMMARY AND OBJECTS OF THE INVENTION

An object of the present invention is to provide a method of producingpolysilicon of a desired surface roughness, said method providingimproved consistency and control.

Another object of the present invention is to provide a method ofdetecting the conversion of amorphous silicon to rough polysiliconduring said conversion.

Still another object of the present invention is to provide a method ofdetecting the conversion of rough polysilicon to smooth polysiliconduring said conversion.

Still another object of the present invention is to provide a method offorming polysilicon having a desired surface roughness by detecting thedesired surface roughness during the formation thereof and stopping theformation process upon formation of the desired surface roughness.

In accordance with the method of the present invention, polysilicon of adesired surface roughness is reliably and repeatably formed by providinga layer of doped or undoped amorphous silicon on a substrate and heatingthe substrate while monitoring the emission of the substrate relative toan expected emission attributable to the heating regime employed. Anincrease in the monitored emission not attributable to the heatingregime signals the transition from amorphous silicon to roughpolysilicon. A decrease of the monitored emission not attributable tothe heating regime signals a transition to smooth polysilicon. Theincrease and decrease in the monitored emission can be used to end theheating regime at the time at which the desired surface roughness ofpolysilicon is formed, or merely to passively monitor the process.

The in situ monitoring of the present invention provides the advantagesof greater control, reliability, and repeatability in formingpolysilicon having a desired surface roughness and electrical and othercharacteristics related thereto.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained may be more fully explained, amore particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 is a schematic representation of some materials and equipmentuseful in the method of the present invention.

FIG. 2 is a graph of an emission of seeded amorphous silicon measured bya single color pyrometer over time.

FIG. 3 is a graph of a reflectivity curve of the amorphous siliconresulting after the processing represented in FIG. 2.

FIG. 4 is a graph of a measured emission of seeded amorphous siliconover time, beginning with amorphous silicon and converting to roughpolysilicon.

FIG. 5 is a graph of a reflectivity curve of the amorphous siliconresulting after the processing represented in FIG. 4, with processingterminating in a region A.

FIG. 6 is a graph of a reflectivity curve of the rough polysiliconresulting after the processing represented in FIG. 4, with processingterminating in a region C.

FIG. 7 is a graph of a measured emission of amorphous silicon over time,beginning with amorphous silicon and converting to smooth polysilicon.

FIG. 8 is a graph of a reflectivity curve of the silicon produced afterthe processing represented in FIG. 7, with processing terminating afterthe end of a region C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an improved method for formingpolysilicon having a particular desired crystal structure and surfaceroughness. The present invention is particularly useful in reliablyproducing hemispherical grain (HSG) polysilicon for improvedcapacitance.

According to a preferred embodiment of the method of the presentinvention, an amorphous silicon layer is provided on a substrate. Theamorphous silicon layer may be doped or undoped, and implanted ifdesired. The amorphous silicon layer may be produced by any suitablemeans including CVD using a precursor such as Si₂ H₆, and PVD, Thesubstrate is then heated to a preselected temperature and seeded for alength of time, then heated to and maintained at a second temperature ofat least 100° C., where the second temperature is preferably within therange of about 500 to 1000° C. The emission of the substrate with theamorphous silicon layer thereon is measured at a wavelength somewherewithin the range of about 10⁻² to 100μm, and is preferably within therange of about 1 to 10μm. The second temperature may optionally be thesame as the seeding temperature.

The means employed to heat the amorphous silicon can be any conventionalmeans including rapid thermal anneal (RTA) devices and conventionaldevices for providing deposition environments, such as CVD, PECVD, andLPCVD reactors and the like. The amorphous silicon may be heated in avacuum or at ambient pressure. Typically known means other than seedingmay also be used to induce crystal formation. Whatever heating means isemployed, the temperature is preferably controlled by a temperaturecontrol means independent of any temperature feedback from the substrateor amorphous silicon layer itself, although feedback temperature controlmay optionally be used, as described hereinafter.

While the amorphous silicon layer is maintained at the secondtemperature, the emission of the amorphous silicon layer is monitored bysuitable means, such as a single color pyrometer. As the amorphoussilicon transforms to rough polysilicon (HSG) or to smooth polysilicon,the curve traced over time by the monitored emission undergoes specificchanges. These changes allow the detection of the transition fromamorphous silicon to rough polysilicon or from amorphous silicon tosmooth polysilicon. This detection capability may be used merely tomonitor a process for forming polysilicon, or the output of the emissionmeasuring device may be coupled directly into a controller for closedloop feedback control of the surface roughness formation process. Withfeedback control, inherent process variability can be minimized oravoided by stopping the process and cooling the polysilicon at themoment a desired surface roughness is reached.

FIG. 1 shows some equipment and materials useful in the method of thepresent invention. A substrate 12 in the form of a silicon wafer has hada layer of amorphous silicon deposited on both the front and back sidesthereof Substrate 12 and the amorphous silicon layer are heated bysuitable means, and crystal formation is induced by suitable means. Anemission detector 20 in the form of a single color pyrometer is placedso as to detect emissions from the back side of substrate 12. Front sideemission detection may also be used. Emission detector 20 may be coupledto a controller 22 for closed loop feedback process control.

FIG. 2 shows a measured emission curve E measured by a single colorpyrometer arranged as illustrated in FIG. 1. The substrate with a layerof amorphous silicon thereon was first seeded for 20 seconds at a firsttemperature of 630° C., then annealed for 4 minutes at a secondtemperature of 685° C. Measured emission curve E is shown on a y-axisscale of degrees Celsius according to the calibration of the singlecolor pyrometer, with the x-axis representing seconds.

Measured emission curve E of FIG. 2 is flat or nearly flat during thetime the substrate is held at the second temperature. Measured emissioncurve E of FIG. 2 is thus characteristic of the heating regime employed.No changes appear in measured emission curve E which are not accountedfor by the heating regime.

FIG. 3 shows a measured percent reflectance curve R, as a function ofwave length, of an amorphous silicon layer after processing according tothe heating regime represented in FIG. 2. An upper characteristic curveU and a lower characteristic curve L for the reflectance of smoothpolysilicon are also displayed. The x-axis is scaled in nanometers (nm).Reflectance curve R between 200 and 400 nm shows the characteristics ofamorphous silicon, demonstrating that the processing according to theheating regime of FIG. 2 did not convert amorphous silicon into rough orsmooth polysilicon.

FIG. 4 is another graph of a measured emission curve E measured by asingle color pyrometer arranged as in FIG. 1. A substrate having anamorphous polysilicon layer thereon was seeded for 30 seconds at a firsttemperature of 660° C. and then annealed for 80 seconds at a secondtemperature of 685° C. At the beginning of the anneal at the secondtemperature, measured emission curve E, in region A thereof, is the sameas the characteristic curve expected due to the temperature regimealone. But in region B of measured emission curve E, the measuredemission increases. In region C, the measured emission remains at thehigher level reached in region B. The increase in the measured emissionat region B has no corresponding increase in the characteristic emissioncurve, which is the emission expected from the heating regime employed.The emission change measured in region B is related to the formation ofrough polysilicon, as shown below.

FIG. 5 is a graph of the measured reflectance of an amorphouspolysilicon film processed according to the heating regime illustratedin FIG. 4, but with the processing not completed as shown in FIG. 4, butterminated instead in region A of measured emission curve E. Reflectancecurve R of FIG. 5 is characteristic of amorphous silicon, showing thatthe amorphous silicon layer remains amorphous during region A of FIG. 4.

FIG. 6 is a graph of the measured reflectance of an amorphouspolysilicon film processed according to the heating regime illustratedin FIG. 4, with the processing terminated in region C of FIG. 4. Theextremely low reflectance exhibited between 200 and 400 nm ischaracteristic of rough polysilicon or HSG, showing that by the time ofregion C of the measured emission curve of FIG. 4, the amorphous siliconlayer has been converted into rough polysilicon.

FIG. 7 is yet another graph of a measured emission curve E measured by asingle color pyrometer arranged as in FIG. 1. A substrate having anamorphous polysilicon layer thereon was seeded for 5 seconds at a firsttemperature of 710° C. and then annealed for 45 seconds at a secondtemperature identical to the first, 710° C. At the beginning of theanneal, measured emission curve E, in region A thereof, is the same asthe characteristic curve expected due to the temperature regime alone.But in region B of measured emission curve E, the measured emissionincreases. In region C, the measured emission decreases.

The increase in the measured emission at region B has no correspondingincrease in the characteristic emission, which is the emission expectedfrom the heating regime employed. Likewise, the decrease region C has nocorresponding decrease in the characteristic emission expected from theheating regime employed. The emission change measured in region B isrelated to the formation of large grained polysilicon, as noted above,while the emission changes measured in region C are related to theformation of small grained or smooth polysilicon, as shown below. Adecrease such as that in region C may also be observed without any priorincrease.

FIG. 8 is a graph of the measured reflectance of an amorphouspolysilicon film processed according to the heating regime illustratedin FIG. 7, with the processing completed as shown in FIG. 7. Reflectancecurve R of FIG. 8 is characteristic of smooth polysilicon, demonstratingthat the amorphous silicon layer undergoes a transition during region Cof FIG. 7 from the rough polysilicon formed in region B to smoothpolysilicon.

The increases and decreases in the measured emission illustrated abovemay be used to detect the formation of the desired smoothness orroughness of polysilicon, upon which the heating of the substrate andpolysilicon layer may be terminated. Alternatively, the measuredemission may be monitored only as a control to evaluate the process,without direct feedback. Other heating regimes than those above may beemployed.

The exact mechanisms for causing the characteristic increase in emissionduring the transition to rough polysilicon illustrated above have notbeen determined. The increase in emission seen in region B may be duemostly to an increase in emissivity, or to increased emissivity and anactual increase in temperature. Both the emissivity and the absorptioncharacteristics of a material can change with changes in the surfacethereof. It is thus possible that an increase in both temperature, dueto increased absorption, and an increase in emissivity may occur. Whilea single color pyrometer is currently preferred for monitoring theemission changes, other instruments, including dual or multi-colorpyrometers may be employed.

While a heating mechanism employing environmental temperature control ispresently preferred, rather than one employing feedback temperaturecontrol using feed back from the substrate, a feedback temperaturecontrol system may nonetheless be employed in the present invention. Itis preferred that the feedback temperature control be sensitive toactual temperature only, and not to emissivity changes, while theemission detector employed is sensitive to all emission changes. Evenwith a constant temperature maintained at the substrate by a feedbackcontroller, the emissivity changes would then be detected by theemission detector as an increase in emission from the substrate,signalling the formation of rough polysilicon.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A method for forming polysilicon having a desired surfaceroughness, said method comprising:depositing amorphous silicon upon asubstrate; heating said substrate; monitoring an emission of saidsubstrate while heating said substrate; and terminating said heatingafter a time determined in response to said emission; said emissionbeing characteristic of the achievement of said desired surfaceroughness.
 2. The method as defined in claim 1, wherein monitoring saidemission of said substrate comprises using a single-color pyrometer tomonitor said emission of said substrate within a wavelength range fromabout 10⁻² μm to about 100 μm.
 3. The method as defined in claim 2,wherein monitoring said emission of said substrate comprises using asingle-color pyrometer to monitor said emission of said substrate withina wavelength range from about 1 μm to about 10 μm.
 4. The method asdefined in claim 2, wherein said substrate is a silicon wafer having abackside and said single-color pyrometer monitors said emission of saidwafer from said backside of said wafer.
 5. The method as defined inclaim 1 wherein heating said substrate comprises RTA.
 6. The method asdefined in claim 1, wherein terminating said heating after said time isconducted at formation of at least some rough polysilicon from saidamorphous silicon.
 7. The method as defined in claim 1, whereinterminating said heating after said time is conducted after formation ofrough polysilicon from said amorphous silicon.
 8. The method as definedin claim 1, wherein terminating said heating after said time isconducted at formation of at least some smooth polysilicon.
 9. Themethod as defined in claim 1, wherein terminating said heating aftersaid time is conducted after formation of smooth polysilicon.
 10. Amethod for forming polysilicon having a desired surface roughness, saidmethod comprising:forming amorphous silicon upon a substrate; heatingsaid amorphous silicon upon said substrate; monitoring an emission ofsaid substrate while heating said amorphous silicon upon said substrate;and terminating said heating in response to said emission; said emissionbeing characteristic of the achievement of said desired surfaceroughness.
 11. A method for detecting the conversion of amorphoussilicon to polysilicon having a surface roughness, said methodcomprising:providing a layer of amorphous silicon on a substrate;heating said substrate; measuring an emission of said substrate whileheating said substrate; and detecting a first level of said emissionrepresentative of conversion of at least a portion of said layer ofamorphous silicon to polysilicon having said surface roughness.
 12. Themethod as defined in claim 11, further including, after detecting saidfirst level of said emission, detecting a second level of said emissionrepresentative of conversion of substantially all of said layer ofamorphous silicon to polysilicon having said surface roughness.
 13. Themethod as defined in claim 11, further including, after detecting saidfirst level of said emission, detecting a progressive decrease in saidemission representative of the onset of formation of smooth polysilicon.14. The method as defined in claim 13, further including, afterdetecting said progressive decrease in said emission, detecting an endof said progressive decrease in said emission representative ofsubstantial completion of said formation of smooth polysilicon.
 15. Themethod as defined in claim 11, wherein heating said substrate comprisesRTA.
 16. The method as defined in claim 11, wherein measuring saidemission of said substrate comprises using a single-color pyrometer tomonitor said emission of said substrate, said emission being within awavelength range from about 10⁻² μm to about 100 μm.
 17. The method asdefined in claim 16, wherein measuring said emission of said substratecomprises using a single-color pyrometer to monitor said emission ofsaid substrate, said emission being within a wavelength range from about1 μm to about 10 μm.